leetcode刷题。总结，记录，备忘162

leetcode162Find Peak Element

A peak element is an element that is greater than its neighbors.

Given an input array where num[i] ≠ num[i+1], find a peak element and return its index.

The array may contain multiple peaks, in that case return the index to any one of the peaks is fine.

You may imagine that num[-1] = num[n] = -∞.

For example, in array [1, 2, 3, 1], 3 is a peak element and your function should return the index number 2.

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Note:

Your solution should be in logarithmic complexity.

使用二分查找。找中点，如果比左右两数大，返回。如果左边的数大于中点，选择左半边，右边的数大于中点，选择右半边。如果中点为0或者是倒数第二个数，直接做一个比较即可。

class Solution {public:    int findPeakElement(vector<int>& nums) {        int beg = 0, end = nums.size()-1;            while (beg < end)        {            int mid = (beg + end) / 2;            if (mid == 0)            return nums[0] > nums[1] ? 0 : 1;            if (mid == end)            {                return nums[end] > nums[end-1] ? end : end-1;            }            if (nums[mid] > nums[mid -1] && nums[mid] > nums[mid + 1])            return mid;            else if (nums[mid] < nums[mid + 1])            beg = mid + 1;            else if (nums[mid] < nums[mid - 1])            end = mid - 1;        }                return beg;    }};